The Vikings were venturesome seafarers. From Denmark, Norway and Sweden they spread through Europe and the North Atlantic in the period of vigorous Scandinavian expansion known as the Viking Age.

Although they are often thought of primarily as raiders, the Vikings were also traders, explorers and settlers. Behind them they left a legacy not only of archaeological remains, but also of family names, place names and fieldnames. Their "remains" can be found in local dialects and customs, in folk tales and oral traditions, and of course in the genetic make up of the local people themselves.

A Viking raid nearly always came as a total surprise. The Vikings were expert navigators and knew the coasts of Europe like the back of their hand. They decided on a target and planned the attack in advance. Having the fastest ships of the day they would arrive seemingly out of nowhere close to the target and storm ashore ready to attack

If needed, they could row their ships up rivers and if the target was some distance from the river they would leave their ships, round up available local horses and head for the church, convent or village of their choice. They often had superior weapons and were well trained, and usually got what they wanted. After the raid they would return to their ships and sail away

For the people attacked it was difficult to organise a defence, because of the speed with which everything happened.

http://viking.no/e/travels/eraid.htm